chapter iii: loaders and linkers

3. Loaders & Linkers1

Chapter III: Loaders and Linkers

Chapter goal: To realize how a source

program be loaded into memory Loading Relocation Linking

Overview: Design of an absolute loader The Bootstrap loader Relocation Program linking Algorithm for a linking loader Other loader design options


Loading Bring the object program into memory for execution.

Relocation Modifies the object program so that it can be loaded at

an address different from the location originally specified.

Linking Combines two or more separate object programs and

supplies the information needed to allow references between them.

Introduction


Loader is a system program that performs the loading function.

Linker perform the linking operations.

P.s. 1. Many loader also support relocation and linking. P.s. 2. Some system have a linker to perform the linking

operations and a separate loader to handle relocation and loading.

Introduction (cont.)


No need to perform functions as linking and program relocation.

The operation is very simple.

All functions are accomplished in a single pass.

Basic loader functions – The absolute loader


The Object code for the program in @2.1


The Object code for the program in @2.1 (cont.)


The object program for the program in @2.1


Loading of an absolute program


Algorithm for an absolute loader


Basic loader functions – Bootstrap loader

When a computer is first turned on or restarted, a special type of absolute loader, called a bootstrap loader, is executed.

The bootstrap loader itself begins at address 0 in the memory of the machine.

It loads the operating system (or some other program) starting at address 80.


Bootstrap loader for SIC/XE


Bootstrap loader for SIC/XE (cont.)


Machine-dependent loader features

The advantages and disadvantages for absolute loader Advantage: Simple and efficient. Disadvantage:

Need the programmer to specify the actual address at which it will be loaded into memory. It is difficult to use subroutine libraries efficiently.

So we need --- the capability of program relocation.


Machine-dependent loader features --- Relocation

Loaders that allow for program relocation are called relocating loaders or relative loaders.

The relocating loaders need to take care of the modification record which must be changed when the program is relocated.


Example of a SIC/XE program


Example of a SIC/XE program (cont.)


Object program with relocation by modification records.


Relocation (cont.)

In SIC/XE machine, most of the code is not need to relocation (e.g., PC relative addressing or immediate addressing). Only the Format 4 (e.g, +SUB…) needed.

Since the SIC machine does not support the PC relative mode, so lots of the instructions need to be relocated.

The SIC machine use bit mask to indicate, whether the relocation is needed or not.

E.g., bit mask FFC (=1111 1111 1100) means the first 10 words need to be relocated.


Relocatable program for a standard SIC machine


Relocatable program for a standard SIC machine (cont.)


Object program with relocation by bit mask.


Program Linking


Program Linking (cont.)


Object program corresponding to above program


Object program corresponding to above program (cont.)


External symbol table (ESTAB)


Relocation an linking operations performed on REF4 from PROGA


Program from above program after linking and loading


Algorithm and Data structure for a Linking Loader

Two passes of a linking loader Pass1 Assigns addresses to all external symbols Pass2 Performs the actual loading, relocation, and linking.


Algorithm and Data structure for a Linking Loader (cont.)


Enhancing the object program

Reference number To indicated each external symbol referred to in a

control section. Reference number 01 is corresponding to the control

section name.


Enhancing the object program (cont.)


Machine-independent loader features – Automatic library search

Automatic library call (library search) Libraries such as mathematical or statistical routines.

The subroutines called by the program being loaded are automatically fetched from the library, linked with the main program, and loaded.

The programmer does not need to take any action beyond mentioning the subroutines names as external references in the source program.


Machine-independent loader features – Loader options

A special command language that is used to specify options.

E.g., INCLUDE program-name(library-name) DELETE csect-name CHANGE name1,name2

INCLUDE READ(UTLIB) INCLUDE WRITE(UTLIB) DELETE RDREC, WRREC CHANGE RDREC, READ CHANGE WRREC, WRITE


Loader design options – Linkage Editors

A linking loader performs all linking and relocation operations, and loads the linked program directly into memory for execution.

A linkage editor produces a linked version of the program (loader module or executable image) which is written to a file or library for later execution.


Loader design options – Linkage Editors (cont.)


Loader design options – Dynamic linking

The linkage editors perform linking operations before the program is loaded for execution.

The dynamic linking (dynamic loading or load on call) load a subroutine and linked to the rest of the program when it is first called.


Loader design options – Dynamic linking (cont.)

chapter iii: loaders and linkers

Documents

sicxe program cont

system program

machinerelocatable program

program linkingprogram

programobject program

source program

relocation cont

machine cont